The present invention relates to a spreading code selection process for wireless communications systems such as CDMA systems. The present invention relates, in particular, but is not necessarily limited, to third generation TDD, and FDD cellular systems, data systems therefore and very high speed downstream internet access CDMA systems.
Cellular wireless has enjoyed an extremely rapid growth from the 1980s. There is now an almost widespread coverage of cellular radio services in industrialised countries. In recent years there has been a similar, albeit more rapid growth in the demand for data services from systems such as the internet, intranet and other data transmission systems. People have become familiar with the advantages of both mobile wireless voice communications and web speed data communications. A demand is growing for mobile wireless data services. The demand for such services is illustrated by the extent of e-mail retrieval, web browsing etc. that already takes place in such locations as airport lounges, hotel lobbies, company conference rooms, etc.
Data traffic is assymmetric, in contrast with that voice traffic. A far greater data rate is necessary for a down link from a wireless access point to a subscriber terminal than that required for of the reverse link (or uplink).
One proposed wireless data system Is the High speed data (HSD) system which provides a high speed, high capacity wireless technologies compatible with CDMA networks for data services. It is intended that the HSD system will require minimal network and spectrum resources. The proposed HSD system provides a shared resource architecture rather than a circuit switched system whereby network, spectrum and air-link resources are minimised.
HSD follows on from existing CDMA technologies such as the IS-95 systems. The RF characteristics of IS-95 are examined whereby operators may make evolutionary changes to their existing IS-95 system. Existing network may be retained, i.e. components cost levels may be reduced by the continued use of existing technology components and devices. Some HSD proposals decouple data services from voice services: rather than providing an equal grade of services to all users. HSD proposes to allocate each user a maximum data rate possible, dependant upon application requirements and wireless channel conditions.
In the TDD mode of UTRA, as standardised in the 3rd generation partnership (3GPP) the downlink channels can use one code with a spreading factors of 1, 2, 4, 8, 16. The parallel spreading factors are orthogonal and FIG. 1 represents an orthogonal variable spreading factor (OVSF). This allows the possibility of mixing the spreading factors if a user has several different channels open simultaneously. Prior to this the SF would be fixed at 16. Control signals will use their own fixed spreading factors, irrespective of the SF chosen for the data.
A further proposal is that certain mixed spreading factors can be employed whereby a xe2x80x9cmulticodexe2x80x9d extension can be employed which allows one user to have several code words in order to offer a fine grained set of bit rates.
In the situation where all branches of the orthogonal spreading factor (OVSF) tree are in use, given that the simultaneous use of high and low SF""s on the same branch is forbidden due to non-orthogonality, the optimum downlink receiver is essentially the zero-forcing (ZF) filter or its MMSE variant. If some branches of the OVSF tree are not in use then the ZF solution is suboptimum and higher CNR can be developed by using a ZF solution optimised for the few spreading codes with the highest common SF. Block based solutions to the uplink problems in TDD are essentially the same as the down-link case though each user has a different channel impulse response which Increases the work load.
It has been shown that the uplink FDD component of CDMA can be equalised and multiple access interference cancelled using what is essentially a TDD algorithm. In order to do this it was assumed that the short scrambling codes option would be used and the block equations are written out with a block size equal to the scrambling code length of 256 bits, which, whilst giving a high work load, is practicable. With some numerical Improvements to some known methods it becomes possible to consider applying a least squares linear solution for the long scrambling code FDD uplink option also. The resources available for solving least squares linear equations are matrix inversion, Cholesky triangularisation and back substitution, block FFT""s, circulants and pseudoinverse methods. However there are unlimited ways for combining theses methods and for factoring and partitioning of the sets of equations in order to get down to the irreducible matrix which must be inverted.
Code words are selected from the orthogonal variable spreading factor (OVSF) tree which allows code words of different length to be mixed yet remain orthogonal. If a code passes through a node A in this tree ten no other shorter code word can be used which passes through node A. Thus when the code word C4,0={1,1,1,1} is in use, another user would be allowed to use word C4,1={1,1,xe2x88x921,xe2x88x921} which Is orthogonal but another user would not be allowed to use word C2,0{1,1} which, if repeated twice: {1,1},{1,1}, would not be orthogonal to C4,0.
Equalisation in terminals is common for the TDD mode of CDMA as it enhances BER performance and can increase the spectral efficiency. Low cost DSP equalisers are available for TDD systems. There are several types of equaliser which have low work load and are suitable for use in battery powered terminals but which can only provide a restricted DSP capability due to battery power drain considerations. These are:
(i) Cholesky factorisation of the channel impulse response autocovariance matrix. This factorisation is efficient when the autocovariance matrix is strongly banded (associated with low dispersion channels). After matrix factorisation the least squares filter equations are solved by back substitution as usual in the Cholesky method.
(ii) Decision feedback equaliser (DFE). This uses a combination of forward FIR filtering, a threshold decision device, and a feedback filter and is commonly used in FDMA applications such as the US 2nd generation cellular phone receivers and telephone modem equalisers.
(iii) Zero-forcing filter. Here an FIR fitter is synthesised which equalises the channel dispersion for a finite time span about the origin. For an n-tap FIR filter solution to a set of linear equations will always form a weight set such that the convolution of the sample channel impulse response hk and the filter wk will have a combined impulse response which is zero at nxe2x88x921 arbitrary points and has unit response at k=0.
(iv) The Wiener least squares filter. This is a modified inverse filter which controlled the white noise response of the filter, ie. the undesired enhancement of thermal noise from the antenna. If the discrete frequency response of the channel is Hk and the thermal noise variance is "sgr"2 then the Wiener filter frequency response is       W    k    =            H      k      *                                "LeftBracketingBar"                      H            k                    "RightBracketingBar"                2            +              σ        2            
A Fast Fourier transform can be used to form this solution with low average work load and this technique is disclosed in a separate patent application, filed concurrently with this application.
In general least squares solutions such as (i) and (iv) are better than algebraic solutions such as zero forcing solutions and give better BER performance. The optimal least square filter is a function of the number of code words in use and their spreading factors. For example a LS filter selected to be optimum for use with a subset of the codes, as is common in 2nd generation CDMA systems, is different from one which is optimum for the case that all codes are in use at the same time. It Is also much more complicated to compute the filter coefficients for use with a code subset and a matrix solution similar to (i) as is necessary for one in which all the spreading codes are explicitly written into the mathematical equations. However it can be shown that when all codes are in use the simpler Wiener filter (iv) is an optimum solution. Presently there exists a problem associated with selecting codes and the complexity associated with equalisation which requires expensive processing power and of course has attendant potential power consumption issues.
The present invention seeks to provide an Improved method of selecting codes for equalisation in communication systems where variable orthogonal spreading factors are employed. The present invention also seeks to provide an associated equaliser for CDMA communications systems.
In accordance with a first aspect of the Invention, there is provided a communications system wherein signal transmissions are subject to orthogonal variable spreading factors which factors enable codes to be selected and which factors can be described by an orthogonal variable spreading factor (OVSF) tree characterised in that a subset of spreading codes may be employed in a transmission, which subset of codes pass through a common node In the OVSF tree.
Conveniently the signals can be transmitted In time division duplex mode (TDD) or frequency division duplex mode (FDD). The communications system is operable to provide data, and/or voice signals.
In accordance with a second aspect of the Invention there is provided a synchronous CDMA transmitter wherein signal transmissions are subject to orthogonal variable spreading factors which factors enable codes to be selected and which factors can be described by an orthogonal variable spreading factor (OVSF) tree characterised in that subset of spreading codes may be employed in a transmission, which subset of codes pass through a common node in the OVSF tree.
In accordance with a third aspect of the invention, there is provided a method of operating a CDMA wireless communications system wherein signal transmissions are subject to orthogonal variable spreading factors which factors enable codes to be selected and which factors can be described by an orthogonal variable spreading factor (OVSF) tree characterised in that a subset of spreading codes are employed in a transmission, which subset of codes pass through a common node In the OVSF tree. Thus a subset is used, instead of the full set, to reduce the bit rate under bad conditions or to reduce mutual interference between users.
The signals can be transmitted in time division duplex mode (TDD) or frequency division duplex (FDD) mode. The communications system can be operable to provide data, and/or voice signals.
In accordance with a fourth aspect of the invention, there is provided an integrated chip programmed to operate in accordance with the method of the present invention.
The present invention, thereby allows the use by the cellular system of a subset of equal length codes which pass through a common node in the OVSF tree and an associated simple equaliser.